Driver fatigue and unused seat belts were among multiple factors that contributed to a fatal crash involving comedian Tracy Morgan and a Walmart truck, killing one person and injuring others, say US transport regulators.

National Transportation Safety Board (NTSB) officials, in a public hearing to review the 7 June 2014 crash on the New Jersey Turnpike, said the vehicle carrying Morgan had been customised, leaving few ways for people to escape in an emergency.

The board cited driver fatigue, noting that the Walmart truck driver had been awake for more than 28 hours at the time of the crash and had driven overnight to the company's distribution centre before embarking on his delivery.

"His drive from Georgia to Delaware took place overnight, causing him to miss a sleep cycle. And the crash occurred at 12.55 am, a time when humans are biologically predisposed to sleep. These factors led staff to believe the driver was fatigued and that this fatigue resulted from acute sleep loss and excessive time awake," said Dennis Collins, NTSB senior accident investigator.

All of the passengers in the rear compartment of the van carrying Morgan were not wearing seat belts, the board said. NTSB investigators also recommended that Walmart review available data on its drivers and delivery operations.

Morgan, who suffered a serious head injury and broken bones and was left in a coma, and the family of his friend who died in the crash, comedian James "Jimmy Mack" McNair, have settled with Walmart, although terms have not been made public. Morgan said in June that he was still recovering from the accident but planned to return to comedy soon.

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